The essence of seismic isolation bridge is to separate the structure from the ground motion which may cause damage as much as possible. For this purpose, the structural seismic force can be reduced by prolonging the fundamental period of the structure to avoid the range of seismic energy concentration. Because the restoring force models for anti-seismic devices are nonlinear, it can only be analyzed by the nonlinear direct integral time-history method. The multi-mode response spectrum method is first proposed in Specifications for Seismic Design of Highway Bridges (JTG/T 2231-2020). This method can solve the problems such as lack of corresponding seismic safety assessment results, limited seismic wave data, low efficiency of time-history analysis. Based on the specification and the actual engineering case, the paper expatiates multi-mode response spectrum method and offers explanations of iterative process involved in the calculation.